The Capitol actually has two domes. The outer dome was scaled to the size of the exterior of the building, whereas the smaller interior dome is sized to the rotunda’s proportions. Thirty-six iron trusses support both domes.
The Capitol Dome
Domes soar to great heights and span vast spaces–their inspiring form is reserved for society’s greatest buildings.
The Capitol’s iron dome, an instantly recognizable American symbol, has long been admired for its majestic beauty and its ingenious engineering.
Designed by Thomas U. Walter, the dome was influenced by classical European domes, such as St. Paul’s in London, St. Isaac’s in St. Petersburg, and the Panthéon in Paris. Capt. Montgomery C. Meigs of the U.S. Army Corps of Engineers supervised much of its construction. It was especially challenging to build because it replaced an existing wooden dome (completed by Charles Bulfinch in 1824), but re-used the old rotunda walls as foundations.
Technical difficulties were easy to overcome compared to those of the Civil War, which broke out just six years after the new dome was begun. When the contractors continued installing ironwork despite wartime conditions, President Abraham Lincoln viewed the rising dome as a sign that the Union would continue as well.